l6o THE DESICCATION OF AFRICA. 



would a way through the mountains be suspected by anyone 

 approaching to within even a few miles of the poorts. 



It is the general rule among geographers to make the 

 Tasili-Tibesti-Marra Hills line a great water divide for this 

 part of Africa, but there is no real reason for maintaining this. 

 I would rather divide North Africa into three partitions, served 

 by three northwards-flowing rivers, the Proto-Niger, the Proto- 

 Congo and the Proto-Nile. 



Besides the fact that the gradient of the rivers is inclined 

 from Lake Chad towards the Ennedi Hills, which is similar to 

 the case of Lake Faguibine on the Niger, with the depression 

 near Taodeni, 300 feet below the level of the lake, there is a 

 further parallel between the Niger and the Chad systems. At 

 Taodeni, there are vast deposits of rock-salt, filling in synclines 

 of the older rocks and often outcropping on the surface. To 

 the east of the Djourab depression, on the other, that is, the 

 east side of the Ennedi Hills, there are the salt mines of Domi. 

 In the case of Taodeni, we ascribed the salt deposits to the 

 drying up of an inland lake, like the Great Salt Lake of Utah; 

 the same applies to the salt deposits of Domi. The water to 

 fill such a lake must have come from the Lake Chad direction, 

 for the Tibesti Highlands could never of themselves have 

 aff^orded sufficient moisture to fill it, seeing that they are 

 entirely isolated by immense stretches of desert. Further, we 

 find east and north of Tibesti the great depression of the 

 Libyan Desert, filled with shifting sand-dunes; there is still 

 abundant underground moisture, however, which comes to the 

 surface in the Kufra Oases and, further north, in the Oasis of 

 Augila. 



The Tibesti Highlands culminate in the volcanic peak of 

 Tusidde (7,800 feet). Nachtigal found the valleys on both 

 sides of the range, with flowing water and with apes, gazelles 

 and birds to enliven the scenery. The moisture to maintain 

 this state of affairs is drawn from the depressions of the 

 Djourab, Bodele and Chad, and is precipitated by the cold 

 surface of the high peaks of the Tibesti range. The rivers 

 now disappear in the sand at the foot of the hills, and what 

 we now see in the shape of animal and plant life is a relic of 

 a time when the whole district was not only much more moist, 

 but was connected by fertile plains with the rest of the habit- 

 able parts of Africa. 



Until an actual outlet to the east has been found for the 

 Bahr-el-Ghazal, our arguments cannot amount to more than a 

 surmise, but the evidence seems sufficiently strong to indicate 

 that the Congo once flowed north along the Gribingi and Shari 

 Rivers to Lake Chad. If that was so, then there must have 

 been an outlet through the gap, we suppose exists, between the 

 high granite hills to the east, a gap which apparently is barred 



